Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ap121_defconfig b/configs/ap121_defconfig
index bd3d0da..afe6c96 100644
--- a/configs/ap121_defconfig
+++ b/configs/ap121_defconfig
@@ -3,6 +3,7 @@
 CONFIG_ARCH_ATH79=y
 CONFIG_DEFAULT_DEVICE_TREE="ap121"
 CONFIG_DEBUG_UART=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_F=y